What do the regions of a Pourbaix diagram mean?

Immunity — the metal is thermodynamically stable and will not corrode. Corrosion — a soluble ion is stable, so the metal dissolves. Passivation — a protective solid oxide/hydroxide is stable, which may (but is not guaranteed to) protect the metal. The diagram tells you which is thermodynamically favoured at a given potential and pH.

Why are some lines horizontal, some vertical, some sloped?

Horizontal lines are reactions involving electrons but not H⁺ (potential-dependent, pH-independent). Vertical lines involve H⁺ but not electrons (pH-dependent only). Sloped lines involve both, and their slope follows the H⁺-to-electron ratio through the Nernst equation.

Can a Pourbaix diagram tell me the corrosion rate?

No. It is purely thermodynamic — it shows what is stable, not how fast anything happens. A region marked "corrosion" might corrode slowly, and a "passivation" region only protects if the film is actually dense and adherent. Use kinetics (polarisation, rate models) for rates.

Corrosion · guide

Reading a Pourbaix (E–pH) diagram

A Pourbaix diagram is a thermodynamic map of corrosion: it shows, for every combination of electrode potential and pH, whether a metal is safe, dissolving, or hiding behind an oxide film. It is the first thing to reach for when reasoning about a new aqueous environment.

Three regions

Pourbaix divided the E–pH plane into three behaviours. Immunity: the bare metal is the stable species, so it cannot corrode. Corrosion: a dissolved ion is stable, so the metal goes into solution. Passivation: a solid oxide or hydroxide is stable and may form a protective film. Where you sit is set by the potential (how oxidising the environment is) and the pH.

The Nernst equation sets the lines

Every boundary is an equilibrium between two species, and its position follows the Nernst equation. At 25 °C:

E = E° − (0.0592 / n) · log Q

For a reaction consuming m protons and n electrons, this gives a boundary with slope −0.0592·(m/n) volts per pH unit — which is exactly why the lines tilt the way they do.

The water stability window

Two dashed lines bound the region where water itself is stable: the lower (hydrogen evolution) and upper (oxygen evolution) lines. Real aqueous corrosion happens between them, because outside that band water is decomposing rather than the metal reacting.

Standard diagrams are drawn for a chosen ion activity (often 10⁻⁶ M as the threshold for "corrosion"). Change the assumed concentration and the corrosion/passivation boundaries shift.

Open the calculatorPourbaix (E–pH) diagram tool →Plot the E–pH diagram for a metal–water system with the immunity/corrosion/passivation regions and water stability lines.

What it is good for

Use a Pourbaix diagram to reason about cathodic protection (push the potential down into immunity), anodic protection (push it up into a stable passive film), the effect of acidifying or alkalising an environment, and why a metal that is passive at neutral pH can actively corrode in acid. Just remember it answers "is it stable?", never "how fast?".

Frequently asked

What do the regions of a Pourbaix diagram mean?: Immunity — the metal is thermodynamically stable and will not corrode. Corrosion — a soluble ion is stable, so the metal dissolves. Passivation — a protective solid oxide/hydroxide is stable, which may (but is not guaranteed to) protect the metal. The diagram tells you which is thermodynamically favoured at a given potential and pH.
Why are some lines horizontal, some vertical, some sloped?: Horizontal lines are reactions involving electrons but not H⁺ (potential-dependent, pH-independent). Vertical lines involve H⁺ but not electrons (pH-dependent only). Sloped lines involve both, and their slope follows the H⁺-to-electron ratio through the Nernst equation.
Can a Pourbaix diagram tell me the corrosion rate?: No. It is purely thermodynamic — it shows what is stable, not how fast anything happens. A region marked "corrosion" might corrode slowly, and a "passivation" region only protects if the film is actually dense and adherent. Use kinetics (polarisation, rate models) for rates.

References

M. Pourbaix, "Atlas of Electrochemical Equilibria in Aqueous Solutions," NACE.
D.A. Jones, "Principles and Prevention of Corrosion," Prentice Hall.
E. McCafferty, "Introduction to Corrosion Science," Springer.
ASM Handbook Volume 13A, "Corrosion: Fundamentals, Testing, and Protection."

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